Mechanical alloying is a solid-state powder processing technique involving repeated cold welding, fracturing, and re-welding of powder particles in a high-energy ball mill. Mechanical alloying is capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or pre-alloyed powders, and finds particular application in the production of oxide-dispersion strengthened nickel- and iron-base superalloys. The basic mechanism of mechanical alloying is repeated deformation, fracture and cold welding by high energetic ball collisions. Depending on the dominant process during milling, such as fracturing, welding or micro-forging, a particle may become smaller through fracturing or may grow through agglomeration. It is a means for producing composite metal powders with controlled, extremely fine microstructures, and can be used to produce alloys that are difficult or impossible to produce by conventional melting and casting techniques.
Mechanical alloying typically begins by blending individual powder constituents which may have diameters ranging from 1 to 500 μm. Additional constituents may include powdered intermetallic compounds. Initial ball-powder-ball collision causes the ductile metal-powders to flatten and work harden, and severe plastic deformation increases the particle's surface-to-volume ratio and ruptures the surface films of adsorbed contaminants. More brittle constituents are more severely comminuted. At the completion of the mechanical alloying process the powder particles generally have an extremely deformed metastable structure and may include dispersoids of the brittle material, depending on initial powder composition.
Mechanical alloying is used to produce coating feedstock because of the ease with which a fine admixture of constituents can be formed, including relatively homogeneous dispersions of refractory oxides. As a result, mechanical alloying lends itself to the fabrication of oxide-dispersion strengthened (ODS) alloys. ODS alloys utilize yttrium oxide evenly distributed throughout the matrix to maintain strength at higher temperatures than conventional superalloys, and have been under intensive development over the past four decades. However, a key factor that limits their wide usage is the high manufacturing cost. The dissolution of Y2O3 is difficult and requires a long time of milling, and insufficient dissolution of yttrium oxide will lead to an inhomogeneous microstructure even after post-mill processing. Generally, milling times in excess of 60 hours are necessary in order to generate adequate yttrium oxide dispersion. See e.g., Klueh et al., “Oxide dispersion-strengthened steels: A comparison of some commercial and experimental alloys,” Journal of Nuclear Materials, Vol. 341 (2-3) (2005); see also Zbiral, “Tensile Properties of Mechanically Alloyed/Milled ODS-Ni-Based Alloys,” Metallurgical and Materials Transactions A, Vol. 27A (1996), among others. It would be advantageous to provide a process whereby a small grain, composite particle comprised of intermetallics and strengthening dispersoids could be generated with significant reduction of necessary milling times. It would be particularly advantageous if milling times necessary for the fabrication of ODS alloys were effectively reduced. Additionally, most mechanically alloyed powders are consolidated by hot compaction followed by hot extrusion, or by direct hot extrusion at temperatures greater than half of their melting point, and the extruded bars are then thermomechanically processed to desired grain structures or cold rolled to sheet. It would be additionally advantageous to provide a fabrication process whereby composite particles generated by a reduced milling time could be applied and consolidated through spray application followed by heat treatment. Such an application technique would greatly mitigate the processing steps generally required between production of ODS composite particles and the final state of an oxide-dispersion strengthened alloy.
Additional challenges associated with ODS alloys involve general shaping and joining concerns. Because ODS alloys are usually manufactured by mechanical alloying followed by hot-extrusion or hot isostatic pressing, these alloys are mostly fabricated as massive pieces, and large scale ODS superalloy sheets are very difficult to obtain by rolling due to the poor ductility of ODS alloys. Additionally, joining techniques which preserve the microstructure and intrinsic strength of ODS alloys are severely limited, which limits their ability to be incorporated into load bearing structures. Fusion welding, commonly used for metals, is not ideal for ODS alloys because melting disrupts both the fine, uniform dispersion of nano-sized oxide particles and the coarse grain structure of recrystallized ODS alloys. This limits the extent to which ODS alloys can be joined in order to generate geometric features that might be advantageous, such as cooling channels in a high temperature turbine vanes, among others. It would be advantageous to provide composite particles that could be applied in a manner such that internal features could be constructed using a removable fugitive phase in conjunction with cold spray application, so that such internal features could be fabricated without dependence on strength-impacting joining technologies.
Provided here is a method for the fabrication of small grain, composite particles comprised of intermetallics and strengthening dispersoids providing a significant reduction in necessary milling times. The methodology has particular applicability to the production of ODS composite particles. Additionally, the fabrication methodology provides composite particles suitable for cold spray application followed by heat treatment, greatly mitigating the processing steps typically employed between composite particle production and final fabrication. Additionally, the methodology allows construction of internal features using a removable fugitive phase in conjunction with cold spray application, so that such internal features can be fabricated without dependence on strength-impacting joining technologies.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.